User terminal and radio communication method

ABSTRACT

A user terminal includes a receiver that receives, from a radio base station, indication information to indicate whether or not uplink signals of a plurality of user terminals are non-orthogonal-multiplexed; a processor that switches between transmission power control methods based on the indication information; and a transmitter that transmits an uplink signal by using a switched transmission power control method.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of and, thereby,claims benefit under 35 U.S.C. § 120 to U.S. patent application Ser. No.14/914,914 filed on Feb. 26, 2016, titled, “RADIO BASE STATION, USERTERMINAL AND TRANSMISSION POWER CONTROL METHOD” which is a nationalstage application of PCT Application No. PCT/JP2014/070730, filed onAug. 6, 2014, which claims priority to Japanese Patent Application No.2013-180275 filed on Aug. 30, 2013. The contents of the priorityapplications are incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a radio base station, a user terminaland a transmission power control method that are suitable for futureradio communication systems.

BACKGROUND ART

In UMTS (Universal Mobile Telecommunications System), which is alsoreferred to as “W-CDMA (Wideband Code Division Multiple Access),” codedivision multiple access (CDMA) is used as a radio access scheme. CDMAis a radio access scheme that does not provide orthogonality withincells. Consequently, in UMTS, transmission power control (TPC) isexecuted in order to reduce the multiple access interference(interference between users within cells, interference within cells,etc.) that accompanies the near-far problem.

Also, in the uplink in LTE (Long Term Evolution), single carrierfrequency division multiple access (SC-FDMA) is used as a radio accessscheme (see, for example, non-patent literature 1). SC-FDMA is a radioaccess scheme that provides orthogonality within cells. Also, in LTE,link adaptation is carried out, such as scheduling per transmission timeinterval (TTI) that is one msec long, adaptive modulation and coding(AMC) and so on. Consequently, in LTE, unlike W-CDMA, it is notnecessary to execute transmission power control for reducing theinterference between users within cells.

Meanwhile, since LTE is based upon one-cell frequency reuse,interference from nearby cells (inter-cell interference) and thepropagation loss (path loss) between user terminals and radio basestations increase. Consequently, in LTE, transmission power control totake into account inter-cell interference, propagation loss and so on isexecuted in order to fulfill the required received quality with respectto uplink signals (see, for example, non-patent literature 1).

CITATION LIST Non-Patent Literature

Non-Patent Literature 1: 3GPP IS 36.213, V8.8.0 “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical layer procedures”

SUMMARY OF INVENTION Technical Problem

Now, in future radio communication systems referred to as, for example,“FRA (Future Radio Access),” the use of non-orthogonal multiple access(NOMA), which is premised upon canceling interference on the receivingend, as an uplink radio access scheme, is under study.

In non-orthogonal multiple access, uplink signals from a plurality ofuser terminals with varying channel states (for example, varyingpropagation losses, SINRs (Signal to Interference plus Noise Ratios),SNRs (Signal-Noise Ratios), etc.) are superposed (non-orthogonalmultiplexed) on the same radio resource, and transmitted with differenttransmission power. On the receiving end, the uplink signals for desireduser terminals are extracted by cancelling other user terminals' uplinksignals.

However, when non-orthogonal multiple access (NOMA) is used on theuplink, if the above-noted transmission power control for LTE, which isdirected to reducing inter-cell interference, is applied to uplinksignals of a plurality of user terminals that arenon-orthogonal-multiplexed, this might result in a threat that the gainof non-orthogonal multiplexing cannot be optimized.

The present invention has been made in view of the above, and it istherefore an object of the present invention to provide a radio basestation, a user terminal and a transmission power control method,whereby uplink signal transmission power control that is suitable whennon-orthogonal multiple access (NOMA) is used on the uplink can beexecuted.

Solution to Problem

The transmission power control method of the present invention providesan uplink signal transmission power control method, which includes thesteps of, in a radio base station, deciding whether or not tonon-orthogonal-multiplex uplink signals of a plurality of userterminals, transmitting, to a user terminal, switching information tocommand a switch to one of a first transmission power control method,which is used when the uplink signals are non-orthogonal-multiplexed,and a second transmission power control method, which is used when theuplink signals are not non-orthogonal-multiplexed, based on the decisionin the decision section, and transmission power determining information,which is used to determine transmission power of an uplink signal, andin the user terminal, determining the transmission power of the uplinksignal based on the switching information and the transmission powerdetermining information, and transmitting the uplink signal with thedetermined transmission power.

Advantageous Effects of Invention

According to the present invention, it is possible to execute uplinksignal transmission power control that is suitable when non-orthogonalmultiple access (NOMA) is used on the uplink.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram to explain an example of link adaptation on theuplink;

FIGS. 2A and 2B each show a diagram to explain an example ofnon-orthogonal multiple access (NOMA) on the uplink;

FIGS. 3A and 3B each provide a diagram to explain downlink controlinformation that is used in a transmission power control methodaccording to a first example;

FIG. 4 is a sequence diagram to show the transmission power controlmethod according to the first example;

FIG. 5 is a flowchart to show the transmission power control methodaccording to the first example;

FIGS. 6A and 6B each provide a diagram to explain downlink controlinformation that is used in a transmission power control methodaccording to a second example;

FIG. 7 is a sequence diagram to show the transmission power controlmethod according to the second example;

FIG. 8 is a structure diagram of a radio communication system accordingto the present embodiment;

FIG. 9 is a diagram to show an overall structure of a radio base stationaccording to the present embodiment;

FIG. 10 is diagram to show a functional structure of a radio basestation according to the present embodiment;

FIG. 11 is a diagram to show an overall structure of a user terminalaccording to the present embodiment; and

FIG. 12 is a diagram to show a functional structure of a user terminalaccording to the present embodiment.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a diagram to explain an example of link adaptation on theuplink. As shown in FIG. 1, a radio communication system to employ linkadaptation is formed to include a radio base station (eNB: Macro eNodeB)that forms a cell and a user terminal (UE: User Equipment).

In the radio communication system shown in FIG. 1, the user terminaltransmits a sounding reference signal (SRS) on the uplink (step S1).Using the sounding reference signal, the radio base station measures theuplink channel state (for example, the SINR (Signal to Interference plusNoise Ratio), the SNR (Signal-Noise Ratio), the RSRQ (Reference SignalReceived Quality) and so on, also referred to as “channel gain”) (stepS2).

Also, based on the measured channel state, the radio base stationallocates the user to radio resources (for example, resource blocks(RBs)) (scheduling). Also, based on the measured channel state, theradio base station determines the modulation scheme and coding rate(adaptive modulation and coding (AMC)).

The radio base station transmits radio resource allocation information(RB assignment), modulation and coding scheme information (MCSassignment) to represent the modulation scheme and coding rate,retransmission control information (HARQ-related signals) and so on, byusing a downlink control channel (PDCCH: Physical Downlink ControlChannel, EPDCCH: Enhanced Physical Downlink Control Channel, etc.) (stepS3).

The user terminal transmits an uplink shared channel (PUSCH: PhysicalUplink Shared Channel) by using the radio resource represented by theallocation information from the radio base station and by using themodulation scheme and coding rate represented by the modulation andcoding scheme information (step S4).

In this way, a radio communication system which executes link adaptationis able to follow instantaneous fading fluctuations by means of adaptivemodulation and coding (AMC), so that there is no need to executehigh-speed transmission power control (for example, transmission powercontrol per several milliseconds to several tens of milliseconds).Meanwhile, in order to fulfill the required received quality withrespect to uplink signals, it is necessary to execute transmission powercontrol taking into account inter-cell interference and propagationloss.

So, in a radio communication system to execute link adaptation,transmission power control for uplink signals (for example, theabove-mentioned uplink shared channel, an uplink control channel (PUCCH:Physical Uplink Control Channel), reference signals (for example, theSRS), etc.) is executed by combining open-loop control and closed-loopcontrol. Open-loop control is executed based on parameters reported fromradio base stations in a comparatively long cycle and the propagationloss measured by user terminals.

On the other hand, closed-loop control is executed base on TPC(Transmission Power Control) commands reported from radio base stationsin a comparatively short cycle. Note that TPC commands are determinedbased on channel states between user terminals and radio base stations(for example, the SINR, the SNR, the RSRQ, etc.). Also, TPC commands mayassume values that are determined based on the difference between theaverage received SINR that is averaged in a radio base station over anaveraging period t and the target received SINR.

Uplink signal transmission power control combining open-loop control andclosed-loop control in this way is also referred to as “fractionaltransmission power control (TPC).” In fractional TPC, for example, thetransmission power of an uplink shared channel (PUSCH) in a subframe iis determined by an equation 1:

where P_(CMAX) is the maximum transmission power of the user terminal.Also, M_(PUSCH) is the transmission bandwidth. Also, P_(O) _(_) _(PUSCH)is the target received power when the propagation loss is 0. Also, α isa fractional TPC weighting coefficient. PL is the measurement value ofpropagation loss in the user terminal. Also, Δ_(TF) is an offset tocorrespond to MCS (modulation scheme and coding rate), and may be 0.f(i) is a correction value by a TPC command.

According to fractional TPC, in open-loop control, the PL term ischanged and the target received power is configured depending on theuser terminal's propagation loss. To be more specific, the targetreceived power for user terminals in cell-edge parts is configured low,and the target received power for user terminals in mid-cell parts isconfigured high. Consequently, according to the open-loop control of theabove equation 1, inter-cell interference can be reduced. Note that theparameters in the above equation 1 may be changed as appropriate.

Now, the use of non-orthogonal multiple access (NOMA) as an uplink radioaccess scheme is under study. FIG. 2 is a diagram to explain an exampleof NOMA on the uplink. FIG. 2A illustrates a case where a user terminal(UE) 1 is located in a middle part of a cell (hereinafter referred to asa “mid-cell part”) that is formed by a radio base station (eNB), andwhere a user terminal (UE) 2 is located in an edge part of the cell(hereinafter referred to as an “cell-edge part”). In FIG. 2A, thepropagation loss in the cell increases from the mid-cell part towardsthe cell-edge part. Consequently, in the radio base station, thereceived SINR from the user terminal 2 is lower than the received SINRfrom the user terminal 1.

In uplink NOMA, a plurality of user terminals with varying channelstates (for example, varying propagation losses, SINRs, SNRs and so on,also referred to as “channel gain”) are multiplexed over the same radioresource. For example, in FIG. 2A, the user terminals 1 and 2, whichshow different received SINRs in the radio base station, are multiplexedover the same radio resource. The radio base station extracts thedesired uplink signals by canceling interference signals from receivedsignals by means of SIC (Successive Interference Cancellation). To bemore specific, the radio base station decodes the uplink signals fromthe user terminals in descending order of the received SINR, and cancelsthe decoded uplink signals.

For example, if, in FIG, 2A, the uplink signals of the user terminals 1and 2 are non-orthogonal-multiplexed, the received signal y at the radiobase station can be represented by an equation 2:

y=h ₁√{square root over (P ₁)}x ₁ +h ₂√{square root over (P ₂)}x ₂ +w  (Equation 2)

where x₁ and x₂ represent the uplink signals from the user terminals 1and 2, respectively. Also, P₁ and P₂ represent the transmission powersof the uplink signals from the user terminals 1 and 2. Also, h₁ and h₂represent the channel states between the user terminals 1 and 2 and theradio base station, respectively. Also, w is a predeterminedcoefficient.

As shown in FIG. 2B, the radio base station decodes the uplink signalfrom the user terminal 1 having the higher received SINR, generates areplica of this uplink signal and subtracts this from the receivedsignal y. Next, the radio base station decodes the user terminal 2 withthe lower received SINR, based on the result of subtracting the uplinksignal from the user terminal 1. Note that, in FIG. 2B, R₁ and R₂represent the uplink transmission rates (rates) from the user terminals1 and 2.

In this way, when NOMA is used as an uplink radio access scheme, it isdesirable to adequately control the transmission powers P₁ and P₂ of theuser terminals 1 and 2 that are non-orthogonal-multiplexed, and maximizethe performance indicators of the throughput in cell-edge parts, thethroughput of the whole cell and so on.

However, when NOMA is used as an uplink radio access scheme, if, forexample, transmission power control to use the above equation 1 isapplied to uplink signals of a plurality of user terminals that arenon-orthogonal-multiplexed, there is a threat that the improvement ofsystem performance by non-orthogonal multiplexing cannot be optimized.

To be more specific, in FIG. 2A, according to the transmission powercontrol of above equation 1, the transmission power of the user terminal2 in a cell-edge part is made bigger, and the transmission power of theuser terminal 1 in a mid-cell part is made smaller. However, thetransmission power control of the above equation 1 is designed mainly toreduce the interference against other cells. Since multiple users aremultiplexed in the same cell in NOMA, it is desirable to control uplinktransmission power so that the difference between the received SINRs ofthe uplink signals from the user terminals 1 and 2 in the radio basestation increases, based on the channel gains of the multiple users,rather than reduce the interference against other cells. Consequently,if the transmission power control of the above equation 1 is executed,there is a threat that the improvement of system performance bynon-orthogonal multiplexing cannot be optimized.

So, the present inventors have conceived of preventing the situationwhere the gain of non-orthogonal multiplexing cannot be optimized whenuplink signals of a plurality of user terminals arenon-orthogonal-multiplexed over the same radio resource, by making thetransmission power control method for when uplink signals of a pluralityof user terminals are non-orthogonal-multiplexed and the transmissionpower control method for when uplink signals of a plurality of userterminals are not non-orthogonal-multiplexed different.

With the transmission power control method according to a first exampleof the present invention, a radio base station decides whether or not tonon-orthogonal-multiplex uplink signals of a plurality of userterminals, generates switching information to command a switch to one ofa first transmission power control method, which is used when the uplinksignals are non-orthogonal-multiplexed, and a second transmission powercontrol method, which is used when the uplink signals are notnon-orthogonal-multiplexed, based on the decision, and transmissionpower determining information, which is used to determine transmissionpower of the uplink signal, and transmits the switching information andthe transmission power determining information to the user terminal. Theuser terminal determine the transmission power of an uplink signal basedon the switching information and the transmission power determininginformation, and transmit the uplink signal with the determinedtransmission power.

With the transmission power control method according to the firstexample the present invention, a user terminal switches between thefirst transmission power control method and the second transmissionpower control method for application, based on the switching informationfrom the radio base station, so that it is possible to prevent thesituation where the gain of non-orthogonal multiplexing cannot beoptimized when uplink signals of a plurality of user terminals arenon-orthogonal-multiplexed.

Also, the present inventors have conceived of preventing the situationwhere the gain of non-orthogonal multiplexing cannot be optimized whenuplink signals from a plurality of user terminals arenon-orthogonal-multiplexed, by allowing a radio base station todetermine the transmission power of the uplink signals and report thedetermined transmission power to user terminals, rather than allowingthe user terminals themselves to calculate the transmission power of theuplink signals.

With the transmission power control method according to a second exampleof the present invention, a radio base station determines transmissionpower of an uplink signal and transmits transmission power allocationinformation to represent the determined transmission power to a userterminal. The user terminal transmits the uplink signal with thetransmission power represented by the transmission power allocationinformation.

With the transmission power control method according to the secondexample of the present invention, when uplink signals from userterminals are non-orthogonal-multiplexed, a radio base stationdetermines the transmission power of the uplink signals so that the gainof non-orthogonal multiplexing can be optimized, and reports thedetermined transmission power to the user terminals, so that it ispossible to prevent the situation where the gain of non-orthogonalmultiplexing cannot be optimized.

Now, the transmission power control methods according to the first andsecond examples of the present invention will be described.

FIRST EXAMPLE

The transmission power control method according to the first examplewill be described with reference to FIGS. 3 to 5. With the transmissionpower control method according to the first example, a radio basestation decides whether or not to non-orthogonal-multiplex uplinksignals of a plurality of user terminals. Also, based on the decision,the radio base station transmits, to the user terminals, switchinginformation to command a switch to a first transmission power controlmethod (hereinafter referred to as “NOMA power control method”) for whenthe uplink signals are non-orthogonal-multiplexed, or to a secondtransmission power control method (hereinafter referred to as “OMA powercontrol method”) for when the uplink signals are notnon-orthogonal-multiplexed, and transmission power determininginformation to determine the uplink signals.

Here, when the switching information commands a switch to the NOMA powercontrol method, the transmission power determining information may be apredetermined threshold for the channel states (for example, the SINRs,the SNRs, the RSRPs, etc.) between the user terminals and the radio basestation. On the other hand, when the switching information commands aswitch to the OMA power control method, the transmission powerdetermining information may be a TPC command.

Also, the switching information may be transmitted using a downlinkcontrol channel, or may be transmitted using higher layer signaling suchas RRC signaling. Also, the transmission power determining informationis transmitted using a downlink control channel. Below, a case will bedescribed as an example where the switching information and thetransmission power determining information are transmitted using adownlink control channel.

FIG. 3A shows an example of downlink control information (DCI) that istransmitted using a downlink control channel. As shown in FIG. 3A, inthe transmission power control method using the above equation 1, DCI(for example, DCI formats 0, 3 and 4) includes a TPC command. Forexample, in FIG. 3A, an increase or a decrease of uplink signaltransmission power is commanded by a two-bit TPC command, in four steps.

FIG. 3B shows an example of DCI that is used in the transmission powercontrol method according to the first example. As shown in FIG. 3B, withthe transmission power control method according to the first example,DCI includes the above-noted switching information and transmissionpower determining information. As shown in FIG. 3B, the switchinginformation is formed with one bit, and command a switch to the NOMApower control method or to the OMA power control method with “0” or“1.”For example, “0” commands a switch to the OMA power control method,and “1” commands a switch to the NOMA power control method.

Note that which of “0” and “1” commands a switch to the NOMA powercontrol method or the OMA power control method is by no means limited tothe above as long as it is provided for in the switching rules (whichwill be described later). Also, the number of bits of the switchinginformation is not limited to one bit either.

Also, in FIG. 3B, when the switching information commands a switch tothe OMA power control method, the transmission power determininginformation may be a TPC command (see FIG. 3A). On the other hand, whenthe switching information indicates the NOMA power control method, thetransmission power determining information may be a predeterminedthreshold for the channel states between the user terminals and theradio base station (for example, the SINRs, the SNRs, the RSRPs, etc.).

FIG. 4 is a sequence diagram to show the transmission power controlmethod according to the first example. In FIG. 4, the switchinginformation is transmitted by using a downlink control channel (PDCCH,EPDCCH, etc.), but may also be transmitted using higher layer signalingsuch as RRC signaling.

As shown in FIG. 4, a radio base station reports transmission powercontrol rules, which are the rules for executing transmission powercontrol, and transmission power control parameters, which are theparameters to use in transmission power control, to user terminals (stepS101). For example, the transmission power control rules and thetransmission power control parameters are reported to the user terminalsthrough higher layer signaling such as RRC signaling.

To be more specific, the transmission power control rules include therules for switching between the NOMA power control method and the OMApower control method, and the rules regarding the channel state and apredetermined threshold in the NOMA power control method. For example,the switching rules may provide for commanding a switch to the OMA powercontrol method when the switching information is “0” and commanding aswitch to the NOMA power control method when the switching informationis “1.” Also, the decision rules may provide for applying a transmissionpower P1 when the channel state is better than a predetermined thresholdand applying a transmission power P2 when the channel state is poorerthan the predetermined threshold, in the NOMA power control method. Notethat the transmission power control rules are not limited the rulesdescribed above.

Also, the transmission power control parameters include, as parametersto use in the OMA power control method, the maximum transmission powerP_(CMAX), the target received power P_(O) _(_) _(PUSCH) and theweighting coefficient α in the above equation 1, and so on. Also, thetransmission power control parameters include the above-notedtransmission powers P1 and P2 as parameters to use in the OMA powercontrol method.

The user terminals memorize the transmission power control rules and thetransmission power control parameters that are reported in a memorysection (step S102). The user terminals transmit uplink channel statemeasurement reference signals (for example, SRSs) (step S103).

The radio base station measures the channel states (for example, theSINRs, the SNRs, the RSRPs, etc.) based on the measurement referencesignals from the user terminals, and, based on the measurement results,determines whether or not to non-orthogonal-multiplex uplink signals ofa plurality of user terminals (step S104).

When uplink signals of a plurality of user terminals arenon-orthogonal-multiplexed, the radio base station generates switchinginformation to command a switch to the NOMA power control method, andtransmission power determining information to represent a predeterminedthreshold Th with respect to the channel states. Also, the radio basestation determines the combination of a plurality of user terminals tobe non-orthogonal-multiplexed (user set, UE set, etc.). On the otherhand, when not non-orthogonal-multiplexing uplink signals of a pluralityof user terminals, the radio base station generates switchinginformation to command a switch to the OMA power control method, andgenerates a TPC command as transmission power determining information.

The radio base station transmits DCI, which includes the switchinginformation and transmission power determining information that aregenerated, to the user terminals, by using a downlink control channel(step S105). The user terminals switch between the NOMA power controlmethod and the OMA power control method based on the switchinginformation, and configure the transmission power of uplink signalsbased on the transmission power determining information (step S106).

FIG. 5 is a flowchart to show the operation of the user terminals instep S106 in detail. As shown in FIG. 5, the user terminals determinewhether or not to switch to (whether or not to apply) the NOMA powercontrol method (step S201) based on the switching information (see stepS105 of FIG. 4) and the switching rules (see step S101 of FIG. 4). Forexample, in accordance with the switching rules, the user terminals maydetermine switching to the NOMA power control method when the switchinginformation is “1” and determine switching to the OMA power controlmethod when the switching information is “0.”

When switching to the OMA power control method (step S201: NO), the userterminals configure the transmission power of uplink signals based onthe TPC command provided as transmission power determining information(see step S105 in FIG. 4) (step S202). For example, the user terminalsmay substitute f(i) in the above equation 1 with the correction value bythe TPC command and configure the uplink signal transmission power.

On the other hand, when switching to the NOMA power control method (stepS201: YES), the channel states (for example, the SINRs, the SNRs, theRSRQs, etc.) between the user terminals and the radio base station aremeasured by using downlink measurement reference signals (for example,CRSs: Cell-specific Reference Signals, CSI-RSs: Channel StateInformation-Reference Signals, etc.) (step S203).

The user terminals configure the transmission power of uplink signalsbased on the comparison results of the measured channel states and thepredetermined threshold Th represented by the transmission powerdetermining information. To be more specific, a user terminal decideswhether or not the measured channel state is higher than thepredetermined threshold Th (step S204). Note that this decision in stepS204 is only an example, and different decisions may be made accordingto the decision rules (step S101 of FIG. 4).

When the channel state is better than the predetermined threshold Th(step S204: YES), the user terminal configures the transmission power P1(step S205). In this case, the user terminal is estimated to be locatedin a mid-cell part, so that the transmission power P1, which is lowerthan the transmission power P2 to be described later, is used.

On the other hand, when the channel state is equal to or lower than thepredetermined threshold Th (step S204: NO), the user terminal configuresthe transmission power P2, which is greater than the transmission powerP1 (step S206). In this case, the user terminal is estimated to belocated in a cell-edge part, so that the transmission power P2, which isgreater than the transmission power P1, is used.

As has been described earlier with respect to steps S203 and S204, whenswitching to the NOMA power control method (step S201: YES), the userterminal configures the transmission power of uplink signals based onthe comparison result of the channel state between the radio basestation and the user terminal and a predetermined threshold. Note thatthe transmission powers P1 and P2 are reported in step S101 of FIG. 4 byusing higher layer signaling such as RRC signaling, as transmissionpower control parameters, but this is by no means limiting. Thetransmission powers P1 and P2 ay be reported from the radio base stationto the user terminal by using a downlink control channel. Also, theuplink signal transmission powers P1 and P2 may be configures so thatthe received SINRs in the radio base station vary sufficiently.

As described above, in step S106 of FIG. 4, the user terminal determinesthe transmission power of uplink signals. The user terminal transmitsthe uplink signal with the determined transmission power (step S107).Note that the uplink signals may include an uplink shared channel(PUSCH), an uplink control channel (PUCCH), a reference signal (forexample, SRS) and so on.

With the transmission power control method according to the firstexample, user terminals switch between the NOMA power control method andthe OMA power control method for application based on switchinginformation from a radio base station, so that it is possible to preventthe situation where the improvement of system performance bynon-orthogonal multiplexing cannot be optimized when uplink signals froma plurality of user terminals are non-orthogonal-multiplexed.

(Variation 1)

With the transmission power control method according to the firstexample, when uplink signals of a plurality of user terminals are notnon-orthogonal-multiplexed, transmission power control to usetransmission power correction information (for example, the correctionvalue f(i) in the above equation 1) is executed. With the transmissionpower control method according to variation 1, the transmission powercontrol to use transmission power correction information is executed notonly when uplink signals of a plurality of user terminals are notnon-orthogonal-multiplexed, but also when uplink signals of a pluralityof user terminals are non-orthogonal-multiplexed.

Here, the transmission power correction information is information forcorrecting uplink signal transmission power, and for example, is thecorrection value f(i) by a TPC command in the above equation 1. Thistransmission power correction information varies depending on whether ornot uplink signals of a plurality of user terminals arenon-orthogonal-multiplexed.

To be more specific, when uplink signals of a plurality of userterminals are non-orthogonal-multiplexed, the radio base station reportsan enhanced TPC command to each of a plurality of user terminals thatare non-orthogonal-multiplexed, as transmission power determininginformation. Based on the enhanced TPC commands, the user terminals mayconfigure different correction values f(i) from the correction valuesf(i) for when uplink signals of a plurality of user terminals are notnon-orthogonal-multiplexed.

(Variation 2)

With the transmission power control method according to the firstexample, when uplink signals of a plurality of user terminals arenon-orthogonal-multiplexed, transmission power control is executed basedon comparison results between channel states and a predeterminedthreshold. With the transmission power control method according tovariation 2, transmission power control is executed based on comparisonresults of channel states and a predetermined threshold, not only whenuplink signals of a plurality of user terminals are notnon-orthogonal-multiplexed, but also when uplink signals of a pluralityof user terminals are non-orthogonal-multiplexed.

Note that, with the transmission power control method according tovariation 2, the configuration values of the above-noted transmissionpowers P1 and P2, which are reported as transmission power controlparameters, may vary depending on whether or not uplink signals of aplurality of user terminals are non-orthogonal-multiplexed. Also, thepredetermined threshold for channel states may also vary depending onwhether or not uplink signals of a plurality of user terminals arenon-orthogonal-multiplexed. It is also possible not to report theabove-described switching information. However, the radio base stationsends a report as to whether or not uplink signals of a plurality ofuser terminals are non-orthogonal-multiplexed.

SECOND EXAMPLE

The transmission power control method according to the second examplewill be described with reference to FIGS. 6 and 7. With the transmissionpower control method according to the second example, a radio basestation decides whether or not to non-orthogonal-multiplex uplinksignals of a plurality of user terminals, and determines thetransmission power of the uplink signals based on the decision. Also,the radio base station transmits transmission power allocationinformation to represent the determined transmission power to the userterminals. The user terminals transmit the uplink signals with thetransmission power represented by the transmission power allocationinformation.

In this way, with the transmission power control method according to thesecond example, in both cases where uplink signals arenon-orthogonal-multiplexed and not non-orthogonal-multiplexed, the radiobase station determines the transmission power of the uplink signals andreports this to user terminals. In particular, the radio base stationdetermines the transmission power of uplink signals, based on thedecisions as to whether or not to non-orthogonal-multiplex the uplinksignals, so that the improvement of system performance by non-orthogonalmultiplexing can be optimized. Consequently, when uplink signals from aplurality of user terminal are non-orthogonal-multiplexed, it ispossible to prevent the situation where the improvement of systemperformance by non-orthogonal multiplexing cannot be optimized.

Similar to FIG. 34, FIG. 6A shows an example of DCI that is used intransmission power control methods for LTE and so on. FIG. 6A is thesame as FIG. 3A and therefore will not be described. FIG. 6B shows anexample of DCI that is used in the transmission power control methodaccording to the second example.

As shown in FIG. 6B, with the transmission power control methodaccording to the second example, DCI includes the above-describedtransmission power allocation information. For example, in FIG. 6B, thetransmission power control information is formed with m bits (m≥1).

FIG. 7 is a sequence diagram to show the transmission power controlmethod according to the second example. As shown in FIG. 7, userterminals transmit uplink channel state measurement reference signals(for example, SRSs) (step S301).

The radio base station measures the channel states (for example, theSINRs, the SNRs, the RSRPs, etc.) based on the measurement referencesignals from the user terminals, and, based on the measurement results,determine whether or not to non-orthogonal-multiplex uplink signals of aplurality of user terminals, and determines the transmission power ofthe uplink signals (step S302).

When non-orthogonal-multiplexing uplink signals from a plurality of userterminals, the radio base station determines the combination of theseplurality of user terminals (user set, UE set, etc.), and determines thetransmission power of the uplink signals of a plurality of userterminals so that the improvement of system performance bynon-orthogonal multiplexing is maximized.

On the other hand, when the uplink signals from a plurality of userterminals are not non-orthogonal-multiplexed, the radio base stationdetermines the transmission power of the uplink signals of these userterminals based on the channel states (for example, the SINRs, the SNRs,the RSRPs, etc.) between the user terminals and the radio base station.

The radio base station transmits transmission power allocationinformation to represent the determined transmission power to the userterminals by using a downlink control channel (step S303). The userterminals configure the transmission power represented by thetransmission power allocation information as the transmission power ofthe uplink signals (step S304). The user terminals transmit the uplinksignal with the configured transmission power (step S305).

With the transmission power control method according to the secondexample, when uplink signals from user terminals arenon-orthogonal-multiplexed, the radio base station determines thetransmission power of the uplink signals so that the improvement ofsystem performance by non-orthogonal multiplexing can be optimized, andreports the determined transmission power to the user terminals, andtherefore it is possible to prevent the situation where the improvementof system performance by non-orthogonal multiplexing cannot beoptimized. Also, the radio base station may determine the transmissionpower of the uplink signals itself and report this to the userterminals, so that, although the overhead increases, the transmissionpower of uplink signals transmission can be controlled even moreadaptively.

(Structure of Radio Communication System)

Now, the structure of the radio communication system according to thepresent embodiment will be described below. In this radio communicationsystem, the above-described transmission power control methods accordingto the first and second examples are employed.

FIG. 8 is a schematic diagram of the radio communication systemaccording to the present embodiment. As shown in FIG. 8, the radiocommunication system 1 includes radio base stations 10 (10A and 10B) anda plurality of user terminals 20 (20A and 20B). The radio base stations10 are connected with a higher station apparatus 30, and this higherstation apparatus 30 is connected with a core network 40. Each userterminal 20 can communicate with the radio base stations 10 in cells C1and C2.

In the radio communication system 1, the radio base stations 10 may beeither eNodeBs (eNBs) and transmission points and so on that form(macro) cells, or RRHs (Remote Radio Heads), eNodeBs (eNBs), femto basestations, pico base stations and transmission points and so on that form(small) cells. The user terminals 20 may be mobile terminals or may bestationary terminals. Note that the higher station apparatus 30 may be,for example, an access gateway apparatus, a radio network controller(RNC), a mobility management entity (MME) and so on, but is by no meanslimited to these.

In the radio communication system 1, non-orthogonal multiple access(NOMA) can be used as an uplink radio access scheme. In NOMA, uplinksignals from a plurality of user terminals 20 with varying channelstates (varying SINRs, SNRs, propagation losses, etc.) are multiplexedover the same radio resource. Note that it is equally possible to useorthogonal multiple access such as SC-FDMA as an uplink radio accessscheme.

Also, in the radio communication system 1, non-orthogonal multipleaccess (NOMA) may be used as a downlink radio access scheme, ororthogonal multiple access such as OFDMA (Orthogonal Frequency DivisionMultiple Access) may be used.

Also, in the radio communication system 1, a downlink shared channel(PDSCH), which is used by each user terminal 20 on a shared basis, adownlink control channel (PDCCH), an enhanced downlink control channel(EPDCCH), a PCFICH, a PHICH, a broadcast channel (PBCH) and so on areused as downlink communication channels. Downlink data (including userdata, higher layer control information and so on) are transmitted by thePDSCH. Downlink control information (DCI) is transmitted by the PDCCHand the EPDCCH.

Also, in the radio communication system 1, an uplink shared channel(PUSCH), which is used by each user terminal 20 on a shared basis, aphysical uplink control channel (PUCCH, EPDCCH, etc.), a random accesschannel (PRACH) and so on are used as uplink communication channels.Uplink data (including user data, higher layer control information,etc.) is transmitted by the PUSCH. Also, downlink channel stateinformation (described later), delivery acknowledgement information(ACK/NACK) and so on are transmitted by the PUCCH or the PUSCH.

Also, in the radio communication system 1, cell-specific referencesignals (CRSs), channel state measurement reference signals (CSI-RSs)and so on are used as downlink reference signals. Also, soundingreference signals (SRSs) and so on are used as uplink reference signals.

The structure of a radio base station according to the presentembodiment will be described with reference to FIGS. 9 and 10.

FIG. 9 is a diagram to explain an overall structure of a radio basestation according to the present embodiment. As shown in FIG. 9, a radiobase station 10 has a transmitting/receiving antenna (antenna port) 101,an amplifying section 102, a transmitting/receiving section 103(transmission section and receiving section), a baseband signalprocessing section 104, a call processing section 105 and a transmissionpath interface 106. A plurality of transmitting/receiving antenna 101may be provided.

As for uplink data, a radio frequency signal that is received in thetransmitting/receiving antenna 101 is amplified in the amplifyingsection 102. The amplified radio frequency signal is subjected tofrequency conversion in the transmitting/receiving section 103 andconverted into a baseband signal. This baseband signal is subjected topredetermined processes (error correction, decoding, etc.) in thebaseband signal processing section 104, and then transferred to thehigher station apparatus 30 via the transmission path interface 106.

Downlink data is input from the higher station apparatus 30 to thebaseband signal processing section 104 via the transmission pathinterface 106. In the baseband signal processing section 104, aretransmission control (HARQ (Hybrid Automatic Repeat Request)) process,scheduling, transport format selection, channel coding and so on areperformed, and the result is transferred to the transmitting/receivingsection 103. The baseband signal that is output from the baseband signalprocessing section 104 is subjected to frequency conversion in thetransmitting/receiving section 103 and converted into a radio frequencyband. The frequency-converted signal is then amplified in the amplifyingsection 102 and transmitted from the transmitting/receiving antenna 101.

The call processing section 105 transmits and receives call processingcontrol signals, and manages the state of the radio base station 10,allocates resources, and so on. Note that the processes in the layer 1processing section 1041 and the MAC processing section 1042 may becontrolled by the call processing section 105.

The functional structure of the baseband processing section of the radiobase station according to the present embodiment will be described withreference to FIG. 10. FIG. 10 is a functional block diagram of thebaseband signal processing section 104 of the radio base station 10. Asshown in FIG. 10, the baseband signal processing section 104 has a layer1 processing section 1041, a MAC (Medium Access Control) processingsection 1042, an RLC processing section 1043, a measurement section1044, a decision section 1045 and a generation section 1046.

The layer 1 processing section 1041 mainly performs processes related tothe physical layer. In the layer 1 processing section 1041, processessuch as, for example, channel decoding, a discrete Fourier transform(DFT), demapping, a fast Fourier transform (FFT), data demodulation areapplied to an uplink signal that is received in thetransmitting/receiving section 103. Also, processes such as channelcoding, data modulation, mapping, an inverse fast Fourier transform(IFFT) and so on are applied to an uplink signal that is transmitted inthe transmitting/receiving section 103.

The MAC processing section 1042 applies processes such as MAC layerretransmission control (HARQ) for the uplink signal, uplink/downlinkscheduling, transport format selection for the PUSCH/PDSCH, resourceblock selection for the PUSCH/PDSCH and so on.

For packets received on the uplink/packets to transmit on the downlink,the RLC processing section 1043 divides the packets, combines thepackets, applies RLC layer retransmission control, and so on.

The measurement section 1044 measures the channel state (for example,the SINR, the SNR, the RSRP, etc.) based on a measurement referencesignal (for example, the SRS). The measurement section 1044 estimateschannel state information (CSI) based on the channel state measurementresult. The CSI may include a channel quality indicator (CQI), a rankindicator (RI) and a precoding matrix indicator (PMI: Precoding MatrixIndicator).

The decision section 1045 decides whether or not tonon-orthogonal-multiplex uplink signals of a plurality of user terminals20. To be more specific, the decision section 1045 decides whether ornot to non-orthogonal-multiplex uplink signals of a plurality of userterminals 20 based on channel state measurement results in themeasurement section 1044. When non-orthogonal multiplexing will beexecuted, the decision section 1045 may determine the combination of aplurality of user terminals 20 to he non-orthogonal-multiplexed (userset, UE set, etc.), and outputs this to the generation section 1046.

The generation section 1046 generates transmission power controlinformation for controlling the transmission power of uplink signalsbased on the decision in the decision section 1045. The transmissionpower control information may include one or both of switchinginformation and transmission power determining information (the firstexample and variations 1 and 2), or include transmission powerallocation information (the second example).

To be more specific, in accordance with the first example, thegeneration section 1046 may generate switching information andtransmission power determining information based on the decision in thedecision section 1045. As described earlier, the switching informationcommands a switch to (commands applying) either the NOMA power controlmethod (the first transmission power control method), which is used whenuplink signals are non-orthogonal-multiplexed, or the OMA power controlmethod (the second transmission power control method), which is usedwhen uplink signals are not non-orthogonal-multiplexed.

Also, the transmission power determining information is used todetermine the transmission power of uplink signals in the user terminals20. When the switching information commands a switch to the NOMA powercontrol method, the transmission power determining information may be apredetermined threshold for the channel states between the userterminals 20 and the radio base station 10. Also, when the switchinginformation commands a switch to the OMA power control method, thetransmission power determining information may be a TPC command.

Also, the generation section 1046 outputs the generated switchinginformation and transmission power determining information to the layer1 processing section 1041. Note that the switching information may bemapped to the downlink control channel (PDCCH, EPDCCH, etc.) in thelayer 1 processing section 1041, or may be mapped to the downlink sharedchannel (PDSCH) as higher layer control information for RRC signalingand so on. Also, the transmission power determining information ismapped to the downlink control channel (PDCCH, EPDCCH, etc.) in thelayer 1 processing section 1041.

Also, the generation section 1046 may generate transmission powercontrol rules (the above-described switching rules, decision rules,etc.) and transmission power control parameters (the maximumtransmission power P_(CMAX), the target received power P_(O) _(_)_(PUSCH) and the weighting coefficient α in above equation 1, thetransmission powers P1 and P2 which the user terminals 20 select basedon the comparison results of the channel states and the predeterminedthreshold, etc.). The transmission power control rules and transmissionpower control parameters may be mapped to the downlink shared channel(PDSCH) as higher layer control information for RRC signaling and so on,in the layer 1 processing section 1041.

Also, in the second example, the generation section 1046 determines(allocates) the transmission power of uplink signals based on thedecision in the decision section 1045, and generates transmission powerallocation information representing the determined (allocated)transmission power. When uplink signals from a plurality of userterminals 20 are not non-orthogonal-multiplexed, the generation section1046 may determine the transmission power of the uplink signals based onthe channel states between the radio base station 10 and the userterminals 20. On the other hand, when uplink signals of a plurality ofuser terminals 20 are non-orthogonal-multiplexed, the generation section1046 may determine the combination of the plurality of user terminals 20(user set, UE set, etc.), and determine the transmission power of theuplink signal so that the plurality of user terminals 20 achieves gainfrom the non-orthogonal multiplexing.

The generation section 1046 outputs the generated transmission powerallocation information to the layer 1 processing section 1041. Thetransmission power allocation information is mapped to the downlinkcontrol channel (PDCCH, EPDCCH, etc.) in the layer 1 processing section1041.

The structure of the user terminals according to the present embodimentwill be described with reference to FIGS. 11 and 12.

FIG. 11 is a diagram to show an overall structure of a user terminal 20according to the present embodiment. As shown in FIG. 11, the userterminal 20 has a transmitting/receiving antenna (antenna port) 201, anamplifying section 202, a transmitting/receiving section 203(transmission section and/or receiving section), a baseband signalprocessing section 204, a call processing section 205 and an applicationsection 206. A plurality of transmitting/receiving antenna 201 may beprovided.

Uplink user data is input from the application section 205 to thebaseband signal processing section 204. In the baseband signalprocessing section 204, a retransmission control (HARQ) process,scheduling, transport format selection, channel coding, transmissionpower configuration and so on take place, and the result is transferredto the transmitting/receiving section 203. The baseband signal that isoutput from the baseband signal processing section 204 is subjected tofrequency conversion in the transmitting/receiving section 203, andconverted into a radio frequency signal. The frequency-converted signalis then amplified in the amplifying section 202 and transmitted from thetransmitting/receiving antenna 201.

As for downlink data, a radio frequency signal that is received in thetransmitting/receiving antenna 201 is amplified in the amplifyingsection 202. The amplified radio frequency signal is subjected tofrequency conversion in the transmitting/receiving section 203 andconverted into a baseband signal. This baseband signal is subjected topredetermined processes (error correction, decoding, etc.) in thebaseband signal processing section 204, and then transferred to the callprocessing section 205 and the application section 206. The callprocessing section 205 manages communication with the radio base station10 and so on, and the application section 206 performs processes relatedto higher layers than the physical layer and the MAC layer.

The functional structure of the baseband processing section of the userterminal according to the present embodiment will be described withreference to FIG. 12. FIG. 12 is a functional block diagram of thebaseband signal processing section 204 of the user terminal 20. Thebaseband signal processing section 204 has a layer 1 processing section2041, a MAC processing section 2042, an RLC processing section 2043, anacquisition section 2044, a control method configuration section 2045and a transmission power determining section 2046.

The layer 1 processing section 2041 mainly performs processes related tothe physical layer. In the layer 1 processing section 2041, a downlinksignal is subjected to processes including, for example, channeldecoding, a discrete Fourier transform (DFT), demapping, a fast Fouriertransform (FFT) and data demodulation. Also, an uplink signal issubjected to processes such as channel coding, data modulation, mapping,an inverse Fourier transform (IFFT) and so on.

The MAC processing section 2042 applies MAC layer retransmission control(hybrid ARQ) to the downlink signal, analyzes the scheduling informationfor the downlink (including specifying the PDSCH transport format andspecifying the PDSCH resource blocks), and so on. Also, the MACprocessing section 1082 applies MAC retransmission control to the uplinksignal, analyzes the uplink scheduling information (processes includingspecifying the PUSCH transport format, specifying the PDSCH resourceblocks, etc.), and so on.

For packets received on the uplink and packets received from theapplication section 206 to transmit on the downlink, the RLC processingsection 2043 divides the packets, combines the packets, applies RLClayer retransmission control, and so on.

The acquisition section 2044 acquires the transmission power controlinformation received from the radio base station 10. As described above,the transmission power control information may include one or both ofswitching information and transmission power determining information(the first example and variations 1 and 2), or include transmissionpower allocation information (the second example). Also, the acquisitionsection 2044 may acquire the above-described transmission power controlrules and transmission power control parameters.

The control method configuration section 2045 configures (switchesbetween) the NOMA power control method or the OMA power control methodbased on switching information input from the acquisition section 2044.For example, the control method configuration section 2045 may configurethe OMA power control method when the switching information is “0,” andconfigure the NOMA power control method when the switching informationis “1.” Note that which of “0” and “1” represents the OMA power controlmethod or the NOMA power control method may be provided for in theswitching rules reported from the radio base station 10.

The transmission power determining section 2046 determines thetransmission power of uplink signals, and commands the layer 1processing section 2041 to transmit uplink signals with the determinedtransmission power.

In accordance with the first example, when a switch is made to the NOMApower control method, the transmission power determining section 2046may determine the transmission power of uplink signals based on thecomparison result of the channel state (for example, the SINR, the SNR,the RSRP) and a predetermined threshold represented by the transmissionpower determining information. For example, the transmission powerdetermining section 2046 determines using the transmission power P1 whenthe channel state is better (greater) than a predetermined threshold,and determines using the transmission power P2, which is greater thanthe transmission power P1, when the channel state is poorer (lower) thanthe predetermined threshold. As described earlier, the transmissionpowers P1 and P2 may be reported in advance as transmission powercontrol parameters.

Also, when, in accordance with the first example, a switch is made tothe OMA power control method (when the OMA power control method isapplied), the transmission power determining section 2046 may determinethe transmission power of uplink signals based on a TPC command reportedas transmission power determining information.

Note that, according to variation 1, the transmission power determiningsection 2046 determines the transmission power of uplink signals basedon transmission power correction information (for example, thecorrection value f(i) in the above equation 1) that varies depending onwhether or not the uplink signals are non-orthogonal-multiplexed. Inthis case, the acquisition section 2044 may receive an enhanced TPCcommand from the radio base station as transmission power controlinformation. Also, according to variation 2, too, the transmission powerdetermining section 2046 determines the transmission power of uplinksignals based on the comparison result of the channel state and apredetermined threshold represented by the transmission powerdetermining information. Note that the control method configurationsection 2045 may be skipped in variations 1 and 2, and the above-notedswitching information needs not be reported to the user terminal 20either. However, the radio base station 10 sends a report as to whetheror not to non-orthogonal-multiplex uplink signals of a plurality of userterminals 20.

Also, in accordance with the second example, the transmission powerdetermining section 2046 determines on the transmission power that isrepresented by the transmission power allocation information reportedfrom the radio base station 10. Note that, in the second example, theabove-described control method configuration section 2045 may beomitted.

As described above, the radio communication system 1 according to thepresent embodiment makes it possible to execute uplink signaltransmission power control that is suitable when non-orthogonal multipleaccess (NOMA) is used on the uplink.

To be more specific, with the radio communication system 1, userterminals 20 switch between the NOMA power control method and the OMApower control method for application based on switching information fromthe radio base station 10, so that it is possible to prevent thesituation where the improvement of system performance by non-orthogonalmultiplexing cannot be optimized when uplink signals of a plurality ofuser terminals 20 are non-orthogonal-multiplexed (the first example).

Also, in the radio communication system 1, when uplink signals from userterminals 20 are non-orthogonal-multiplexed, the radio base station 10determines and reports the transmission power of the uplink signals tothe user terminals 20 so that the improvement of system performance bynon-orthogonal multiplexing can be optimized, and therefore it ispossible to prevent the situation where the gain of non-orthogonalmultiplexing cannot be optimized (the second example).

Now, although the present invention has been described in detail withreference to the above embodiment, it should be obvious to a personskilled in the art that the present invention is by no means limited tothe embodiment described herein. The present invention can beimplemented with various corrections and in various modifications,without departing from the spirit and scope of the present invention.That is to say, the descriptions herein are provided only for thepurpose of explaining examples, and should by no means be construed tolimit the present invention in any way.

1. A user terminal comprising: a receiver that receives, from a radio base station, indication information to indicate whether or not uplink signals of a plurality of user terminals are non-orthogonal-multiplexed; a processor that switches between transmission power control methods based on the indication information; and a transmitter that transmits an uplink signal by using a switched transmission power control method.
 2. The user terminal according to claim 1, wherein the receiver receives, from the radio base station, transmission power determining information that is used to determine transmission power of the uplink signal, the processor determines transmission power of the uplink signal based on the indication information and the transmission power determining information, and the transmitter transmits the uplink signal to the radio base station with the transmission power determined.
 3. The user terminal according to claim 2, wherein when the indication information indicates that the uplink signals of the plurality of user terminals are non-orthogonal-multiplexed, the transmission power determining information is a given threshold for a channel state between the user terminal and the radio base station, and the processor determines the transmission power based on a comparison result of the channel state and the given threshold.
 4. The user terminal according to claim 2, wherein when the indication information indicates that uplink signals of the plurality of user terminals are not non-orthogonal-multiplexed, the transmission power determining information is a Transmission Power Control (TPC) command, and the processor determines the transmission power based on the TPC command.
 5. The user terminal according to claim 2, wherein the receiver receives the indication information using higher layer signaling or a downlink control channel, and receives the transmission power determining information using the downlink control channel.
 6. A radio communication method for a user terminal, comprising: receiving, from a radio base station, indication information to indicate whether or not uplink signals of a plurality of user terminals are non-orthogonal-multiplexed; switching between transmission power control methods based on the indication information; and transmitting an uplink signal by using a switched transmission power control method. 